Bug 1606528 - part 1: Implement BrowsingContext::IsTop(). r=kmag

This is a frequent pattern.

Differential Revision: https://phabricator.services.mozilla.com/D58480

--HG--
extra : moz-landing-system : lando

docshell/base/BrowsingContext.h

@@ -221,6 +221,8 @@ class BrowsingContext : public nsISupports,
   bool IsContent() const { return mType == Type::Content; }
   bool IsChrome() const { return !IsContent(); }
 
+  bool IsTop() const { return !GetParent(); }
+
   bool IsTopContent() const { return IsContent() && !GetParent(); }
 
   bool IsContentSubframe() const { return IsContent() && GetParent(); }